The invention is concerned with a doctor roll arrangement comprising a tube revolving on its bedplate, especially for stencil printing.
Doctors in the form of blades or rolls are used for forcing ink through the spaces or fine openings of flat or circular stencils, but also for direct application of viscous compositions onto a surface to be treated, for example the top surface of a moving web.
When using a doctor roll within a silk-screen printing stencil, the intensity of color supplied, per unit of area, to the parts of the web to be printed on depends on the diameter of the doctor roll, the contact pressure thereof, and the viscosity of the color supply. Conditions are similar with doctor rolls that do not work in stencils but apply a viscous mass directly onto the surface to be treated.
Tubular doctors are known for some time (cf. U.S. Pat. No. 2,419,695). They have the advantage that their diameter can be made quite large compared with a solid roll of the same mass. A large roll diameter demands, however, high pressure on that portion of the inking substance that enters a wedge-shaped gap between the doctor roll and the surface on which it rolls. Such a higher pressure leads directly to the deposition of larger quantities of color in stencil printing and the length of the wedge-shaped gap promotes bleeding.
However, in a flat doctor gap only a very limited quantity of ink can enter, and it follows that a narrow limit has hitherto been placed on the augmentation of the ink deposit by use of a tubular doctor. It has now surprisingly been discovered that the ink deposit can be substantially increased if, in accordance with the invention, in the case of a tubular doctor roll, the roll is provided with recesses or gaps through which ink quantities to be deposited can enter from a stock or supply located in front of the tube, to be pressed into the interior of the tube.